1. Field of the Invention
The present invention relates to a gallium nitride-based semiconductor light-emitting device, and more particularly to a gallium nitride-based semiconductor light-emitting device with an improved crystallinity of a gallium nitride-based semiconductor.
2. Description of the Related Art
Generally, a gallium nitride-based semiconductor light-emitting device is a light-emitting device used for obtaining light with a blue or green wavelength, and is made of a semiconductor material having a composition formula of AlxInyGa(1−x−y)N wherein 0≦x,y,x+y≦1. A gallium nitride-based semiconductor crystal layer (hereinafter, referred to as a gallium nitride-based semiconductor layer) may be grown on an heterologous substrate such as a sapphire (α—Al2O3) substrate or SiC substrate. Especially, the sapphire substrate has the same hexagonal structure as gallium nitride, and is primarily used as it is inexpensive and stable at higher temperature, as compared to a SiC substrate.
However, the sapphire substrate also has disadvantages such as a lattice constant difference of about 13% and further a large difference of thermal expansion coefficient (about −34%) from gallium nitride and thus suffers from strain occurring at the interface between the sapphire substrate and gallium nitride single crystals, thereby giving rise to lattice defects and cracks in crystals.
One of conventional strategies to overcome these problems and obtain better single crystals is a heteroepitaxy method involving formation of a buffer layer on the sapphire substrate. A low temperature nucleus-growth layer made of material such as AlxGa1−xN is primarily used as the buffer layer. However, since the low temperature nucleus-growth layer is a polycrystal layer, a gallium nitride-based semiconductor layer formed thereon has been known to have a significant density of crystal defects (level of 109˜1010/cm2). Further, there is required a thermal cleaning process for the sapphire substrate, and processing conditions of growing temperature and thickness of the low temperature nucleus-growth layer are very strict making it difficult to control them within a suitable range, resulting in complex process control and requiring a long processing time.
Alternatively, there is another method involving nitridation of the upper surface of the sapphire substrate followed by growing the gallium nitride-based semiconductor layer. In this method, a gallium nitride-based semiconductor layer with an excellent crystallinity may be grown by improving rough surface conditions of the sapphire substrate to decrease surface energy.
The above-mentioned nitridation is a relatively simple process as compared to the buffer layer technique, but has a significant disadvantage in that it is difficult to grow excellent semiconductor crystals. This defect results from the fact that a gallium nitride-based semiconductor layer grown on a nitridated sapphire surface has a nitrogen-rich surface. This gallium nitride-based semiconductor layer with nitrogen-rich surface has a major polarity appearing in N-polarity (See Japanese Journal of Applied Physics, Vol 36, L73 2000) and materials serving as impurities bind much better to the gallium nitride-based semiconductor layer with an N-polarity surface than to a Ga-polarity surface with a gallium-rich surface. From this, it is known that the gallium nitride-based semiconductor having an N-polarity surface has a decrease in crystallinity as compared to a gallium nitride-based semiconductor layer having a Ga-polarity surface.
Therefore, there remains a need for a process for preparing gallium nitride-based semiconductor light-emitting devices that can employ a crystal film satisfying optimum conditions to grow a high quality semiconductor crystal layer for light-emitting structures in the art.